You ever had a problem that seatbelt is scratching your neck, or in case you have them going over your sensitive parts on the chest? This little thing will allow you to add a forth point of contact on your waist so that belt going over your shoulder can be adjusted :)

Ideally print in ASA as it is going to be in the car exposed to UV light but I printed it in ABS (no support needed, does not warp even on open printer) and it works for over a year without a problem ... I print it with 0.3mm layer 10perimeters (just to make it solid with perimeters, there is no space for 10 of them) but you can do it with .2 or .25 too if you really have time to waste :D